Background:
Platinum (Pt), palladium (Pd), rhodium (Rh) and iridium (Ir) are platinum group elements
(PGEs) and also important elements of geochemistry and environmental chemistry with the similar physic-chemical
properties, which have been widely used in industry and laboratory. However, due to the low abundance and
inhomogeneous distribution in natural ore as well as the nugget effect, the accurate determination of PGEs has been a
challenge to analytical chemistry.
Methods:
In this work, a novel fire assay method was reported for the determination of ultra-trace Pt, Pd, Rh and Ir in
geochemical samples. Tin powder (Sn) instead of stannic oxide (SnO2) was used as fire assay collector to reduce the
melting temperature from 1250 oC to 1050 oC, the escape of molten material caused by high temperature was successfully
avoided. Tin bead was compressed into thin slice and dissolved by HCl. For the target Pt, Pd, Rh and Ir, HCl insoluble
substance such as PtSn4, PdSn4, RhSn4 and Ir3Sn7 were formed and separated from matrix by filtering. The metal
compounds precipitate together with filter paper were microwave-assisted completely digested by aqua regia (50%, v/v),
thence the sample solution were determined by inductively coupled plasma mass spectrometry (ICP-MS).
Results:
Compared with nickel oxide and lead oxide in nickel sulfide /lead fire assay, the reagent blank of tin powder
were relatively low and could be directly employed in tin fire assay to collect Pt, Pd, Rh and Ir without purifying.
Moreover, the harm of nickel oxide and lead oxide to the analyst and environment was avoided by using the non-toxic tin
powder. The decomposition method of chromite and black shale were investigated as well as the amount of tin powder
and flour, microwave digestion program for the determination of Pt, Pd, Rh and Ir were optimized. Besides, the influence
of mass spectrum interference of co-existing elements was discussed and the standard mode and kinetic energy
discrimination collision pool mode were compared. Under the optimal conditions, excellent curve fitting of Pt, Pd, Rh and Ir were obtained between 0.01～100 ng mL-1
, with the correlation coefficients exceeding 0.9996. The detection limits
were from 0.003 ng g
-1
to 0.057 ng g
-1
.
Conclusion:
The developed method was applied to analyze the Chinese Certified Reference Materials and the determined
values were in good agreement with the certified values.
